Drag is the result of skin friction and surface pressure variations induced by viscous effects, especially those due to separation bubbles or regions (i.e., low pressure wakes). Separation regions occur when two and three dimensional boundary layers depart from the surface of the body. Bluff or blunt bodies have shapes which tend to promote a rapidly increasing downstream pressure gradient in the streamline flow around it which can cause the bulk flow to break loose from the surface of the body. This is particularly true for bodies having blunt end surfaces, such as automobiles, tractor trailors, and blunt ended projectiles. The separation bubbles created behind these objects as they move through the air produce high base drag.
Airfoil shaped bodies such as airplane wings, rudders, sails, and gas turbine engine rotor blades and stator vanes have a streamlined shape which, at moderate angles of attack (below about 15.degree.) avoid streamwise two-dimensional boundary layer separation over the entire surface. At higher angles of attack (or increased loading) separation does occur and a recirculating flow region (or a low pressure wake) is formed, greatly increasing drag and reducing lift. As used in the specification and appended claims, "streamwise, two-dimensional boundary layer separation" means the breaking loose of the bulk fluid from the surface of a body, resulting in a flow near the wall moving in a direction opposite the bulk fluid flow direction.
It has been a constant goal of aerodynamicists to reduce the drag and improve lift and stall characteristics (if appropriate) on bodies disposed in a fluid moving relative thereto. A common way to avoid boundary layer separation on an airfoil (or other streamlined body) or to at least delay separation such that it occurs as far downstream along the surface of the airfoil as possible so as to minimize drag, is to reduce the pressure rise downstream such as by tailoring the surface contour along the length of the airfoil in the direction of bulk fluid flow.
Another well known method for reducing the drag on airfoils is to create turbulence in the boundary layer so as to impart a greater average momentum of the boundary layer fluid, which carries it further downstream along the surface against an adverse pressure gradient, thereby delaying the separation point. For example, U.S. Pat. No. 4,455,045 to Wheeler describes elongated, expanding channels in the flow surface. The channels have sharp, lengthwise edges. The boundary layer on the surface flows into the channels, and the channel edges create streamwise vortices below the level of the normal flow surface which energize the flow in the channel to maintain boundary layer attachment of the flow along the floor of the channel.
Similarly, Stephens creates a plurality of adjacent streamwise extending channels in the flow surface. The channels continuously expand laterally from a narrow inlet to a wide outlet. A generally triangular ramp is formed between adjacent channels. Stephens explains that the boundary layer flow is split between the ramps and the channels. The flow within the channels spreads out and the boundary layer becomes thinner and remains attached to the surface longer. The ramp flow is diverted into the general flow. One application (FIG. 6 of Stephens) is between the roof and rear window of an automobile to maintain the flow attached to the curved surface for a greater distance than normal.
In U.S. Pat. No. 1,773,280 to Scott, increased lift without increased drag is created for an aircraft wing by placing a plurality of side-by-side chordwise extending ridges along the top of the wing from its leading to its trailing edge, the ridges having their highest point near the thickest portion of the wing. The ridges themselves are airfoil shaped when viewed from above, tapering to a point at the trailing edge of the wing. This concept does not take into account viscous induced boundary layer separation effects and therefore could not be expected to avoid separation at high lift conditions.
U.S. Pat. No. 3,588,005 to Rethorst uses chordwise extending ridges in the upper surface of an airfoil to delay the onset of separation by providing "channels of accelerated flow in the free stream flow direction to add energy to the boundary layer and maintain laminar flow in the region of normally adverse pressure gradient". The ridges protrude from the surface "to a height of the order of the boundary layer thickness". Cross flow components "are accelerated over the ridges and may reduce the likelihood of separation near the aft end . . . of the body by allowing the flow to `corkscrew` smoothly off the aft end rather than encounter the abrupt adverse pressure gradient in the free stream direction caused by a blunted aft end". As with the ridges of the Scott patent discussed above, flow is also accelerated between the ridges which further helps maintain laminar flow over the airfoil surface.
U.S. Pat. Nos. 3,741,235 and 3,578,264 to Kuethe delay separation by creating vortices using a series of crests or concave depressions which extend substantially transverse to the streamwise flow direction. Kuethe states that the maximum height of a crest or depth of a depression is preferably less than the boundary layer thickness.
In a paper titled "The Reduction of Drag by Corrugating Trailing Edges" by D. L. Whithead, M. Kodz, and P. M. Hield published by Cambridge University, England in 1982, blunt case drag of a blade (having a 20-inch span, 20-inch chord length, a constant thickness of 1.5 inches and a blunt trailing edge) is reduced by forming the last seven inches of its chordwise length into streamwise extending, alternating troughs and ridges (corrugations). The trailing edge and any upstream cross-section across the corrugations has the shape of a sine wave with an 8.0 inch wavelength. The thickness of the blade material is maintained constant over the length of each trough and ridge, although the trough depth or ridge height (i.e., wave amplitude) transitions from a maximum of 2.0 inches at the trailing edge to zero upstream. The total trough outlet area is more than 50% of the blunt base area. FIGS. 21-23 show the blade described therein, with dimensions given in terms of a unit length "a". A reduction of base drag of about one-third was realized when compared with a reference blade without corrugation. It is explained that spanwise vortices which were shed alternately from the top and bottom rear edges of the non-corrugated reference blade were eliminated by the corrugations.
In general, it is believed that the separation delaying devices of the prior art create significant drag in their own right, thereby negating some of the benefits they would otherwise provide. This sometimes limits their effectiveness. While many of the devices of the prior art have proved to be effective in reducing drag, further improvement is still desired, such as with respect to reducing base drag on blunt based objects.